Cathode structure for gas tubes



N 29, 1 E. K. SMITH 2,489,937

CATHODE STRUCTURE FOR GAS TUBES Filed May 15, 1947 3 Sheets-Sheet 1 l A H 5M4 ya f M 1%. W,

W (Ittorneg 2 19 9 E. K. SMITH 2,489,937

CATHODE STRUCTURE FOR GAS TUBES Filed May 15, 1947 3 Sheets-Sheet 2 Zhwento:

MM 7% w (Ittomeg Nov. 29, 1949 E. K. SMITH 2,489,937

GATHODE STRUCTURE FOR GAS TUBES Filed May 15, 1947 3 Sheets-Sheet 3 FIG.7.

LOAD

@ I I I H V i V w) LE 5% L N IE (Ittorneg Patented Nov. 29, 1949 CATHODE STRUCTURE FOR GAS TUBES EarlK. Smith, West Orange, N. 5., assignor to Electrons, Incorporated, Newark, N. .l., a corporation of Delaware Application May 15, 1947, Serial No. 748,160

. Claims.

. V l This invention relates to electron discharge devices, and more particularly to the structure of directlyheated .cathodes for gaseous electron discharge tubes, commonly known as gas tubes,

and more particularly to such cathode structures of the filamentary type. I

In the common type of gas tube, to which this invention relates, the cathode is frequently directly heated to minimize the heating time, and the electron current to the anode from the various points of-the emissive surface of the cathode results in the distribution and flow of load current through the cathode, which acts to raise the cathode temperature. For ordinary applications of gas tubes, however, the heating current alone should be suflicient to render the cathode fully emissive to provide full load current as and when required; and-any heating effect of the load current in the cathode not only represents wasted heat losses, but also has a tendency to overheat the cathode under load conditions and adversely affect the life of the tube.

The cathode for a gas tube is commonly surrounded by a heat shield to conserve heat and reduce the heating current required to obtain the appropriate temperature for the desired electron emission. When such a heat shield is provided, it is convenient to connect the upper end of the filament or filaments constituting the cathode to this heat shield, so as to stabilize its potential, and also provide a convenient connection for supplying heating current to one end of the cathode. In such an arrangement, the heating effect-of the load current becomes substantial, andan important factor in the life of the tube.

Also, in the usual gas tube structure, where one end of the cathode is connected to the heat shield, this connection must carry both the load current and the heating current. When such a connection of the appropriate cross. section to conduct both load current and heating current is pro vided, such'connection becomes a relatively good conductor of heat, and tends to lower the terns perature of the upper portion of the cathode by a direct conduction of heat from this upper portion to the heat shield. Adequate electron emission from the upper portion of the cathode, how ever, is important for initiating a discharge current at low anode voltages; and it is desirable to maintain the upper end of the cathode nearest the anode at normal temperature for adequate electron emission Without overheating other parts of the cathode.

with satisfactory design of a gas tube is that,

when one end of the cathode is connected to the heat shield, the pins of the tube base and cooperating prongsof the tube socket included in the anode circuit have to conduct the load current, aswell as the heating current; and in some instances, particularly for tubes of the higher current ratings, it becomes a problem to provide in the limited space available properly insulated pins and prongs for the tube base and socket which will have sufficient current carrying capacity and heat radiating ability to avoid objectionable overheating of the base or socket. With these and other considerations in mind, the primary object of this invention is to obviate these limitations and disadvantages of the usual type of cathode structure for gas tubes. L Another object of the invention is to provide a simple and effective structural organization for the cathode and heat shield of a gas tube, in which the distribution of load current causes less heating of the cathode, andthe connection from the cathode to the heat shield may be reduced in cross section and obviate its undue cooling efiect upon the upper end of the cathode.

Various other objects, characteristic features and attributes of the invention will be in part apparent, and in part pointed out as the descriptionprogresses. Generally speaking, without attempting to define the nature and scope of the invention, it is proposed to provide a cathode of the double filamentary type having a center tapped connection to the heat shield and with heating current supplied to theseparate ends of the fila-.

ments and flowing through these filaments in series, so that a more effective distribution of load current through the cathode system is obtained, the temperature of the upper portion of the oathode is not substantially reduced by heat conduction to the heat shield, and the current carrying requirements of the pins and prongs of the tube base and socket are reduced. I The accompanying drawings illustrate one specific embodiment of the invention in connection with a grid control gas tube, together with diagrammatic representations of the cathode structure and circuit connections of the usual type and those characteristic of this invention, for the purpose of comparison and explanation.

In these drawings, Fig. 1 is a longitudinal vertical section through the tube;

Fig. 2 is a side view of the tube elements with the envelope in section along the line 2-2 in Another iactonto'be considered in connection Fig. 1;

of the usual type and in accordance with thisinvention.

Although the cathode structure of this invention may be employed in connection with' gaseous:- discharge tubes of any type, with or without control grids, the typical embodiment of the invention shown relates to a grid control tube having an inert gas filling, such as xenon, argon, or the like. The particular type of tube illustrated comprises the usual glass envelope E fused at its lower? end to a circular stem mount Sf provided with the usual exhaust tube I, which is sealed on" after exhaust in the usual manner. The anode A comprises a circular disc; prefer ably of tantalum. with a down turned peripherial flange'a'nd 'a'circular rib 2"or corrugation'fo'r stiffn'ess; This anode A'is welded toa central supporting rod 3 sealed in the upper end of the envelope E and terminating in a cap 4, in ac= c'orda'nce with the usual practice.

The grid of' the tube comprises a plurality of parallel grid bars 6; preferably treated in a manner disclosed in the. prior patent to D. V.' Edwards star, No. 2.012339, August 27, 1935, which are welded across the opening in a flanged ring T at spaced intervals in" accordance with the prior patent' o'f 13. V. Edwards et' al., No: 1,905,692, April '25", 1933. A cylindrical skirt or shield?! c is" welded to th'e flange of the grid ring 1 and the entire grid assembly is supported-bye plurality of supportingelements; three as shown, comprising inverted V-shaiped members I? welded to the grid shield Ill" and rods l3 an: chored'iri the'steni S, one of these rods designated I 30: extending" through the stem S" to provide the e'xter'nm grid" connection;

The cathode assembly comprises a heat" shield HS in the forfn of a cylindricaican, and a cathode in the form of thin corrugated strips in the form of a double Spiral. The heat" shield HS comprises a cylindrical body portion It"; a flanged cover I! welded thereto and having therein a circular: discharge opening l8", and a double walled bottom consisting of two discs I9 and I9 having flanges weldedto the body portion l6, and having openings therein for the insulating sleeves 2c of steatite or the like forthe' tt'vo' sup porting rods 22 for the cathode.

The heat shield HS is supported by a plurality of rods 23', three as shown,. which have their up er ends bent and welded to the inside of the body portion 16' of the heat shield (see Fig; 3), and which are supported at their lower ends in the stem mount s', cheer these rods zaa'e'xtending through the stem to afiord an external connection to the heat shield HS.

In the particular arrangement shown, the cathode comprises two thin corrugated strips 24, preferably of nicke1, which are connected together at their upper ends by a ring or circular collar 25, and are formed in a double interlocked spiral, as shown in Fig. 1. One convenient Way of forming this type of cathode is to start with a blank, as shown in Fig. 4, which may be formed by spot welding together different strips of the 4 appropriate shape and dimension, or as a onepiece stamping as desired.

This blank is then wound on a grooved mandrel, as indicated in Fig. 5, with the ends of the strips held in place by adhesive tape, or other suitable detachable elements. The mandrel is then rolled on a grooved plate or equivalent device to form the desired. corrugationssin. the thin strips by frcing portions thereof into the grooved mandrel. After this corrugating operation, the assembly is removed from the mandrel, and the ends of .the'strip. 25 are welded together to form the upper collar for the cathode assembly.

It is evident that various other expedients, in-

" cludinga' collapsible mandrel, may be employed to perform this fabricating function. The significant feature isthat the cathode or filaments are formed of. thin strips of nickel, say .001 inch thick, which are formed into the desired double interlocked spiral, and have corrugations for adequate stiffness:

The free endsof the strips 24' of' the" cathode assembly after this coiling and 'corrugating operation; are welded to thesupportingrQdsZZ', as"

shown in Fig. 1. The cathode-assembly isthencoated with a suitable emissive' coating; preferably in accordance with the disclosure of the prior patent of D. V. Edwards et'aL, No. 1,985,855, December 25, 1934.

The cathode assembly is'assembled inside the heat shield, with the supportingrods-M extending through theinsulating sleeves 20; and then a bulge or enlargement/of the rods-22= adjacent the lower ends of'the insulating'sl'eeves 20 as indicated at 22a is formed by a suitabledeforming or'welding operationto holdthese insulating sleeves 26 in place. The two-strips or tabs 26 projecting from the upperring 26- (see Fig. 4)- are-then attached to'theheat shield HS, preferablyby inserting and welding these tabs 26-between the flange H and the body portion I6 01' the heat shield HS. Thus,- the upper ring 25 of the cathode assembly is'conneeted by the tabs 26" at two diametrically opposite points, so that the cathode assembly-is definitely-positionedand accurately held in place;-but if desired; one connection from-the upper ring or collar 25 to the heat shield'HS maybe employedi After the cathode and grid assemblies are mounted on the stem S, this stemis sealed to the bottom of the'glassenvelopeE; andthen the tube is subjected to theappropriate exhaust procedure for degassing the tube elements and, envelope, and for activation of the cathode coating, in accordance with the usual practice followed for gas tubes of this type. i

The significance of the structural features of the cathode assembly of this invention can be explained to advantage, by comparing this structure as diagrammatically indicated in Fig. 7;,with the usual type of cathode structure for tubes of this type, as diagrammaticallyrepresented in Fig. 6.

In. the cathode arrangementof; Fig. 6, which is typical of the structures commonly'employed, the cathode comprises a filament F having its upperend connected to the heat shield HS' bya suitable connection, indicated as a connector 30. The lower end of the filament F is connected to a pin in the base, represented at 3|, which engages with the usual prong and socket to afford a connection to one terminal of the secondary of a heater transformer HT. The heat shield HS is connected through another pin 32 and prong to the other terminalv of the secondary of this heater and transformer; and the load circuit, illustrated diagrammatically and including a supply transformer T, is frequently connected to one terminal of the heater transformer secondary, as shown in Fig. 6. Sometimes the load circuit is connected to a center tap of the heater transformer secondary; but when using low heater voltages, in the order of one and one-half volts, such a center tap connection is not easy to obtain, and is sometimes considered objectionable.

Considering the heating effect of the load current for the arrangement shown in Fig. 6, which is taken as typical of prior cathode structures commonly used, it may be assumed for purposes of explanation that, after an arc discharge has been initiated, an ion sheath is formed adjacent the entire emissive surface of the filament, and that electron emission for the existing load current is distributed uniformly over the surface of the filament. Considering the load current as flowing from the anode to the cathode, this load current in amperes per unit area of the emissive surface of the filament may be considered as distributed uniformly throughout the length of the filament, as indicated by dotted arrows in Fig. 6. It can be seen that different portions of the load current flow through the resistance of different portions of the length of the filament. For example, the load current per unit of area represented by the dotted arrow adjacent the lower end of the filament F flows through a resistance equivalent to the whole resistance of the filament, while the load current per unit of area at other points nearer the upper end of the filament flows through less resistance. This heating effect of the load current upon the filament F or equivalent cathode not only represents wasted heat losses and tends to overheat the cathode as a whole under load conditions, as previously explained, but also the variations in the heating effect of the load current for different parts of the length of the cathode makes the overheating uneven and tends to give extreme temperature elevations for certain parts, thereby accentuating the development of hot spots for the filament F, and otherwise adversely affecting the life of the oathode and the tube.

With regard to the amount of the heating of the filament F by the load current, it may be assumed that the average equivalent resistance through which the load current flows is about one-half of the total filament resistance; and the heating efiect corresponds with the square of the load current times one-half of the filament resistance, or

With regard to this prior arrangement represented in Fig. 6, it can also be seen that the load current, as well as all of the heating current, fiows through the upper connector 30 to the heat shield HS. If this connector 30 is made large enough in cross section for satisfactory conduction of both load current and heating current, it assumes dimensions which make it a relatively good conductor of heat, thereby tending to lower the temperature of the upper end portion of the filament by the direction of heat from this upper end portion to the heat shield HS. The temperature and electron emission from the upper end portion of the filament F, however, is a significant factor in the performance of the tube. Prior to the ionization of the gas or vapor in the tube, electron emission from the different portions of the filament F is dependent upon the potential gradient of the anode voltage; and in order that the tube may initiate a discharge at low anode voltages,

full electron emission from the upper end portion of the filament is desirable. When the tem- 5 perature and electron emission from the upper portion of the filament F is thus reduced due to the connector 30, and the starting voltage for the tube is adversely affected, one expedient is to supply more heating current so as to bring the temperature of the upper end portion of the filament to the proper level, in spite of the conduction of heat away from this upper portion by the size of connector 30 used. Such increase in heating current, however, has a disadvantage that it tends to overheat the other parts of the filament which are not subjected to such a cooling eifect; and the overheating of these parts of the filament adversely afiects the tube life.

The cathode structure of this invention is diagrammatically represented in Fig. 7 for comparison and contrast with the usual cathode structure of Fig. 6. It is assumed that the two strips 24 of the cathode structure of this invention as shown in Fig. 1 will afford the same emissive surface as the single filament F in Fig. 6, and will be heated by heating current through the resistance of these strips to a like emitting temperature. The center tap connection between the strips 24 and the heat shield HS, afforded by the ring 25 and tabs 26 shown in Fig. 1, is diagrammatically represented in Fig. 7 for purposes of explanation as a connector 34. If the load current per unit of area of the emissive surface for the cathode structure of this invention in Fig. 7 is assumed to be represented by dotted arrows in the same way as in Fig. 6, it can be seen that the center tap connection characteristic of this invention and represented by the connector 34 in Fig. 7 permits the load current per unit of area of each of the strips 24 to return to the heat shield HS and anode A through the connector 34 without flowing through the other strip 24. In other words, only half of the total load current returns to the anode through each of the strips 24. Assuming comparable operating cathode temperatures and heating currents for the arrangements of Fig. 6 and Fig. 7, the total resistance of the single filament F in the ordinary cathode structure of Fig. 6 is divided between the two strips 24 of the center tapped cathode structure of this invention shown in Fig. 7. Consequently, the total resistance of each strip 24 is only half of the total resistance of the filament F of Fig. 6, and the resistance through which its share of load current flows is equivalent to half of this, or one-fourth of the resistance of the filament F. Thus, for

each strip 24 about one-half of the load current flows through a resistance equivalent to onefourth of the total cathode resistance; and the heating effect of the load current for each strip is the square of half the load current-times one-fourth of the total cathode resistance, or

This makes the total heating efiectof the load current for the two strips equal to as compared with for theusual arrangement shown in Fig. 6. Thus, for cathodes of comparable'resistance,

7 heating current and operating temperatures, the affect of the load current upon the heating and temperature of the cathode with the center tapped cathode structureof this invention is approximately only one fourth of that of the usual type of cathode structure such as represented in Fig. 6. Such substantial reduction in the heating efiect of the load current avoids uneven overheating. of the cathode which, as previously discussed,

adversely affects the useful operating life of the cathodes for gas tubes.

Also, with regard to the cathode structure of this invention represented. in Fig. '7, it can be seen that the heating current flows through the two filaments in series and not through the heat shield HS. In other words, the center tap connection from the two strips 24 of the double spiral filament of this invention to the heat shield is not required to conduct any of the heating current. In tubes of the type under consideration, the heating current is usually large as compared with the load. current. For example, in one type of tube of which I am familiar, which has a direct current continuous rating of 2.5 amperes, the heating current is in the order of 1 2 amperes. Incidentally, this rating of 2.5 amperes average continuous current represents a R. M. S. or heating value of about four ampere's for single phase operation. For convenience, the ratio of heating current to loading current in this type of tube may be assumed to be inthe order of about 3 to 1. Assuming such a ratio of three times as much heating current as load current, it can be readily appreciated that the current carrying requirements for the center tap connector 34 in accordance with this invention is greatly reduced, because it is not required to conduct the relatively large heating current, as well as the load current. This enables the use of a connector 34 of smaller cross section and reduced heating conductivity, thereby decreasing. the tendency to lower the tem perature of the upper end portions of the filament by direct conduction of heat to the heat shield HS.

Also,- in the arrangement of this invention represented in Fig. 7, each of the pins 35 and 3t and associated pl-Ongs is required to conduct heating current, but no load current. A separate pin indicated at 3'! is used to conduct the load current, but this pin is not required to conduct the heating current. In the usual cathode arrangement represented in Fig. 6, the pins 3-! and 32 are re quired to conduct both the heating current and the load current; and it is sometimes difficult to provide adequate conduction for such currents through the bases and sockets of tubes of higher current ratings, which will avoid overheating of the base or socket. Sometimes the requirements of current conductivity call for two pins in multiple for each of the cathode circuit connections; but the space limitations or the tube base and socket make it diificult to support and adequately insulate such additional connections. By way of contrast, it can be seenthat the current requirements for the pins 35 and 36 in the heating circuit, and the pin- 31 in the load circuit, in the tube of this invention represent current requirements which facilitate provision of the appropriate pins and prongs in the tube base and socket for such current conductivity, even for tubes 0 the higher current ratings.

One significant feature of this invention is the use of a plurality of filaments with a center tapped connection to. the heat shield; and while various structures and arrangements may be provided for this purpose, it is preferred to arrange these filaments in a double interlocked spiral as shown. This enables strip filaments of the appropriate width and length for the desired emissive area and: resistance to be brought within satisfactory space limits. Also, such a spiral arrangement of the fiat filaments facilitates heating of the strips to thedesired temperature with low heating currents, on. account of the space relation of the different portions of the filaments and conservation of radiated heat.

From the foregoing, it can be seen that the structure and arrangement of parts constituting a cathode assembly for a gas tube in accordance with this invention serves to reduce the heating effect of the load current, the cooling of the upper end of the cathode, and the current conduction requirements for the pins and prongs of the tube base and socket, each of these characteristics or attributes contributing materially to the efficiency and life of the tube.

Various modifications, adaptations and conditions may be made in the particular structure and arrangement of parts shown and described as a typical embodiment of the invention, without departing from the nature and scope of the invention.

What I claim is:

- 1. A directly heated cathode for gas tubes surrounded by a heat shield and comprising, two

filamentary portions connected together at one end and formed in a double interlocked spiral, and connector means for electrically connecting the connected ends of said filamentary portions to said heat shield, said connector means having a limited cross section to reduce. its cooling eiiect upon the upper portion of the cathode by direct heat conduction to the heat shield.

2. A directly heated cathode structure for gas tubes comprising, a filament formed of an upper member and thin corrugated strips connected at one end to said member and extending therefrom in a double interlocked spiral, said strips having a thermionic emissive coating thereon, means for 45 supplying heating current to the lower ends of said strips, a cylindrical heat shield around said filament, and connector means electrically connecting said upper member of said filament to said heat shield, said connecting means having a restricted cross section to reduce the cooling of the upper end. of the filament by conduction of heat from it to the heat shield.

3. In a directly heated cathode structure for gas tubes, the combination of a cathode comprising a circular collar and two thin strips of nickel extending from diametrically opposite points on said collar in a double interlocked spiral, said collar and strips being formed with corrugations for stiffness and having thereon a thermionic emissive coating, said corrugations extending in planes transversely of the axis of the cathode, a heat shield. around said cathode, and means electrically connecting said collar with said heat shield,- saidv connecting means having a limited cross section to obviate cooling of the upper end of the cathode by conduction of heat to the heat shield.

4. In a directly heated cathode structure for gas tubes, the combination witha cylindrical heat shield, of a cathode within said heat shield comprising a plurality of thin strips of nickel formed in a double interlocked spiral and connected together at their upper ends by a connecting member, said strips being corrugated in planes 7.5 transversely of the heat. shield for circumferential stiffness, means constituting electrical connection of limited cross section between said connecting member of the cathode and said heat shield to reduce the cooling of the upper end of the cathode by conduction of heat to said heat shield, and insulated supporting elements for the lower ends of said strips for conducting heating current through said strips in series.

5. A directly heated cathode structure for gas tubes comprising, in combination with a cylindrical heat shield having a discharge opening in one end, of an oxide coated cathode within said heat shield. and formed from a blank of thin sheet nickel to comprise a circular collar connected to the ends of a plurality of strips in a double interlocked spiral, said collar and strips having stiffening corrugations extending in parallel planes transversely of said heat shield, said collar having radial tabs of limited cross-section connected to the upper end of said heat shield for conducting load current without causing undue cooling of the upper end of the cathode by 5 conduction of heat to the heat shield.

EARL K. SMITH.

REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS 

